BUG in the German tree: "Pos. Pron." and "Nom. Pron." sections are entangled.
Even though each section teaches you different skills (Possessive Pronouns one, and Nominative Pronouns the other one) they are somehow entangled (like entangled particles in physics, which are connected in such a way that actions performed on one of them affect the other one at the same time).
The problem is that whenever you progress through the lessons of one of these sections (let's say Pos. Pron.) you "do so" in the other one (Nom. Pron.), even though you haven't practiced any of the skills contained in that section.
This seems to be independent of the browser. I am using both Chrome and Safari. Always desktop.
I hope you can fix it soon.
It is a known issue. People 1 and 2 are also behaving the same way. Nothing you can do - the developers need to fix it - ot had been reported a long time ago and a lot of trees have the same problem.